This invention relates to conjugated polymers for use in luminescent devices, especially electroluminescent devices, and their synthesis.
Conjugated polymers have been used as organic electroluminescent (EL) materials in suitable device structures, as demonstrated in our earlier patent herein incorporated by reference as U.S. Pat. No. 5,399,502. Poly(p-phenylene vinylene) is one such polymer and may be prepared via the Wessling precursor method as described in, for example xe2x80x9cPrecursor Route to Poly(p-phenylene vinylene): Polymer Characterisation and Control of Electronic Propertiesxe2x80x9d D. D. C. Bradley, J. Phys. D. 20, 1389 (1987). For example, a tetrahydrothiophene-based precursor with a halide counter-ion is typically used as shown in FIG. 1. It has been proposed that the polymerisation of the p-xylenebis(sulphonium halide) monomer occurs via a quinoid intermediate as described in xe2x80x9cThe Polymerisation of Xylylene Bisdialkyl Sulfonium Saltsxe2x80x9d R. A. Wessling, J. Pol. Sci. Pol. Symp. 72, 55-66, (1985). The conjugated polymer formed is insoluble and intractable and therefore the solution processable precursor is required. Device fabrication is carried out using the precursor material and the conjugated polymer is prepared in situ via a thermal conversion step. Typically, the precursor polymer is coated, by spin-coating or blade coating or other coating techniques, onto a transparent conductive oxide layer, for example Indium Tin Oxide (ITO). The ITO is itself coated onto a suitable substrate which may be, for example, glass or plastic. The precursor polymer film is then converted on the ITO by suitable heat treatment. Following this, appropriate metal electrodes are deposited. A multi-layer structure is therefore obtained which consists of an anode, the conjugated polymer, and the cathode. Injection of positive and negative charge carriers at the anode and cathode respectively leads to light emission. Other layers may be included into the device to facilitate charge injection/accumulation or to afford protection during the conversion process. This is shown in FIG. 2.
The advantages of using precursor conjugated polymers as emissive layers in EL devices include:
a) ease of fabrication,
b) amenable to multilayer structures,
c) intractability of converted polymer film, and
d) intrinsic luminescence properties.
However, there is some evidence that the quantum yield for radiative decay of the excited states is lowered through their migration to non-radiative decay centres hence photoluminescence and therefore electroluminescence efficiencies are significantly reduced. Our previous patent , herein incorporated by reference as U.S. Pat. No. 5,401,827 has dealt with this issue by describing a semiconductive conjugated copolymer comprising at least two chemically different repeat units with different semiconductor band gaps (for example conjugated and non-conjugated segments). The optical properties of the copolymer are therefore determined by the relative proportions of the different repeat units. Copolymers were prepared in this work either by copolymerisation of more than one bis(sulphonium) salt, control of the degree of conversion of the precursor polymer, or by the substitution of the THT unit to provide groups that would survive the conversion process. The latter approach is shown in FIG. 3 and will henceforth be referred to as the Substitution Approach.
We now have evidence that conversion of precursor homopolymer or copolymer systems on certain conductive oxide substrates such as ITO, can lead to undesirable interactions that give rise to either quenching of luminescence or to modification of the expected copolymer composition. Furthermore, we have observed that the presence of certain functional groups in the copolymer can be detrimental to device performance, and in particular to device lifetime.
The present invention seeks to provide conjugated arylene vinylene copolymer systems prepared via the precursor approach as emissive layers in EL devices which overcome these difficulties and retain the benefit of enhanced photoluminescence and electroluminescence efficiency.
The present invention provides a process for the preparation of a conjugated poly(arylene vinylene) copolymer for use in a luminescent device, which comprises:
(1) providing a precursor polymer comprising units of general formula 
in which Ar is substituted or unsubstituted arylene, L is a leaving group, R1 and R2 are each independently H, alkyl, alkoxy, aryl or an electron-withdrawing group, and n is an integer;
(2) reacting the precursor polymer with a reactant comprising a carboxylate, an aldehyde, a ketone, a sulphonate, a thioate, a disulphide, a xanthate, an amine, a pyridine, a hydrazide, a phenoxide, an alcohol with a boiling point above 100xc2x0 C., or a derivative thereof, under substitution conditions whereby a proportion of the leaving groups are substituted to form a substituted precursor copolymer comprising units of general formula "Parenopenst"ArCHR1xe2x80x94CR2L"Parenclosest"m"Parenopenst"ArCHR1xe2x80x94CR2X"Parenclosest"l, in which Ar, R1, R2 and L are as defined above, X is a substituent group from the reactant, l and m are independently integers; and
(3) converting the substituted precursor copolymer to a conjugated poly(arylene vinylene) copolymer by elimination of the leaving groups from the substituted precursor copolymer.
Throughout this specification, the term arylene is intended to include in its scope all types of arylenes including heteroarylenes as well as arylenes incorporating more than one ring structure, including fused ring structures. Ar may be paraphenylene, 1,4 naphthylene, 1,4 anthracene, 2,6 fluorene and is preferably paraphenylene.
R1 and R2 may be independently selected from C1-C10 alkyl, C1-C10 alkoxy, aryl such as substituted or unsubstituted phenyl, heterocyclic or polycyclic aryl, xe2x80x94CN or xe2x80x94CF3. Preferably both R1 and R2 are H.
Where the reactant comprises a carboxylate, a carboxylate salt is preferred. In this way, the precursor polymer may be made conventionally such as by base-catalysed polymerisation from suitable monomers such as paraxylylene bis(tetrahydrothiophenium bromide). The carboxylate salt is then added to the precursor polymer. Other suitable monomers are described in U.S. Pat. No. 5,401,827.
Advantageously, the precursor polymer may be provided by base-catalysed polymerisation from suitable monomers in a molar excess of base. In this way, the carboxylate salt is formed by neutralisation of the base with its corresponding carboxylic acid. One advantage of this method is that it can be carried out in a single reaction zone such as a suitable container, in which the precursor polymer is formed in the excess base and, after polymerisation is complete, neutralisation takes place so as to form the substituted precursor copolymer prior to conversion into the conjugated poly(arylene vinylene) copolymer.
The carboxylate may be aliphatic such as formate or acetate and may be formed by the corresponding aliphatic carboxylic acids, formic acid or acetic acid. Substituted or unsubstituted aromatic carboxylates may be used such as those formed from 2,6 dimethylbenzoic acid, as well as derivatives thereof.
Materials of the following general formulae may be used:
Rxe2x80x2xe2x80x94CO2H or Arxe2x80x2xe2x80x94CO2H,
where Rxe2x80x2=(cyclo)alkyl chain, Arxe2x80x2=substituted or unsubstituted aromatic or polycyclic system.
In a further embodiment of this invention the substitution approach may be facilitated by the interaction of precursor polymer with a basic solution of a ketone or aldehyde (ie carbonyl) based system. Typical carbonyl based systems have the general structure shown and would include benzaldehyde, anthraldehyde, and benzophenone:
R3R4C=O or R3HC=O,
in which R3 and R4 are each independently Rxe2x80x2 or Arxe2x80x2 as defined above.
In a further embodiment of this invention oxygen, sulphur and nitrogen nucleophiles may be used to form PPV precursor copolymers. A two-stage synthesis involves precursor polymer preparation followed by treatment with alcohols or alkoxides, thiols or thiolates, or amines( for example diphenylamine) etc. Following this the solution must be neutralised in the normal way using an inorganic acid such as HCl or HBr or by using one of the carboxylic acids listed above.
Oxygen nucleophiles would include sulphonic acids, phenoxide, alcohols, and derivatives thereof. Where alcohols are used they should have a low volatility so that, under thermal conversion to form conjugated copolymer, significant loss of alcohol does not occur.
Sulphur-containing nucleophiles would also include thioacids, disulphides, xanthates, and derivatives thereof.
Nitrogen nucleophiles would include primary, secondary, and tertiary aliphatic or aromatic amines, pyridines, hydrazides, and derivatives thereof.
Converting step (3) is generally carried out at a temperature in the range 80xc2x0 C. to 350xc2x0 C., usually around 150xc2x0 C. at a time in the range 30 minutes to 10 hours, preferably around 4 hours. The process according to the present invention may involve treating the precursor polymer, either before or after purification, with suitable reagents, or carrying out the polymerisation in the presence of such reagents.
Substituent group X is formed from the substitution reaction between the reactant and the precursor polymer, for example by nucleophilic substitution. The reactant may be chosen to produce group X as achemically beneficial moiety to be incorporated into the vinyl copolymer. Thus, group X may be capable of chelating or binding indium or other undesirable impurities such as catalyst residues. Examples of such reactants would include acetyl acetone or 8-hydroxyquinoline. Alternatively, copolymers may be prepared with cross-linkable groups. Such groups would be capable of cross-linking to another part of the conjugated poly(arylene vinylene) copolymer or to another polymer chain such as another chain of the poly(arylene vinylene) copolymer. In this case, substitutent group X would include acrylates and cinnamates and be introduced into the polymer by neutralising the polymerisation solution with acrylic acid or cinnamic acid respectively. The level of incorporation would be determined by the amount of base used for the polymerisation.
In a further aspect of this invention, a low band gap lumophore or chromophore may be incorporated into the copolymer by nucleophilic displacement of the leaving group to yield a semiconductive copolymer containing lower band gap lumophores as compared to the copolymer. Exciton migration to the lower band gap component leads to efficient emission from the lumophore with the corresponding shift in emission spectrum Hence, this method may be used to tune the emission or absorption characteristics of PPV.
An important aspect of the present process is that the concentration of non-conjugated segments in the final copolymer may be increased to such a level that the emission spectrum is significantly blue shifted. In the extreme, blue emission is observed. This method therefore provides a highly controllable way in which to tune the emission from poly(arylene vinylene) copolymers such as PPV copolymers.
In a further aspect, there is provided a process for the production of a luminescent device comprising a conjugated poly(arylene vinylene) copolymer supported on a substrate, which process comprises preparing a conjugated poly(arylene vinylene) copolymer in accordance with the above process wherein converting step (3) is carried out on the substrate.
In a further aspect, there is provided a process for the production of an electroluminescent device comprising a first electrode, a second electrode and at least one layer between the electrodes, including a conjugated poly(arylene vinylene) copolymer layer, which process comprises preparing a conjugated poly(arylene vinylene) copolymer in accordance with the above process, wherein converting step (3) is carried out on the first electrode.
The first electrode may comprise a conductive oxide such as indium tin oxide, aluminium-doped zinc oxide, fluorine-doped tin oxide, vanadium oxide, molybdenum oxide, nickel oxide; a conducting polymer; or a metal film.
In accordance with this invention, the poly(arylene vinylene) copolymers exhibit high photoluminescent efficiencies in the presence or absence of conductive oxide substrates such as indium tin oxide.
According to another aspect of the invention there is provided a multilayer electroluminescent device of high electroluminescence efficiency, incorporating a converted precursor copolymer as the emitting layer and an underlying electrode on which the conversion process has been carried out. At least one other layer is present one of which is the second electrode. The emissive copolymer exhibits enhanced photoluminescence efficiency in the presence or absence of ITO, enhanced EL efficiency and differential stability during device driving.
EL device manufacture is typically carried out by coating of the precursor copolymer by suitable means usually at a thickness of around 100 nm onto, for example, a semi-transparent conductive oxide. The precursor copolymer film is then converted to form the semiconductive conjugated copolymer. Following this, a suitable metal electrode is deposited and, following the application of a suitable voltage, light emission is observed.
The semiconductive conjugated poly(arylene vinylene) copolymers of the present invention may be used as charge transport layers or charge injection layers or light emitting layers in luminescent devices, especially electroluminescent devices including optically or electrically pumped lasers. The polymers may also be used as polymeric fluorescent dyes.
The present invention will now be described in further detail, by way of example only, with reference to the following Examples an he accompanying drawings.